1. Field of the Invention
This invention relates generally to ozone delivery systems, and more specifically to an automatic purging method for use in ozone systems which employ negative pressure to inject ozone into a fluid to be treated.
2. Description of the Prior Art
Ozone is a very powerful gaseous reactant, and its usefulness has been well established for many years in a wide range of industrial applications. Recently its value in all types of water purification applications has been coming to the fore because of its ability to act as a powerful oxidant, microflocculant and disinfectant without producing toxic side-products.
Many ozone systems utilize negative pressure such as from a venturi to draw ozone from an ozone generator into the water to be treated. It would be desirable to provide some means of automatically purging the ozone out of the generator after system shutdown in order to prolong the life of the seals inside the generator (which are usually ozone resistant but not ozone inert). In air, ozone has a half-life of approximately one day, and since in some systems the generator may be on for only an hour at a time, with six hour intervals during which it is off, a purge with air (or oxygen) after each shutdown would prevent the seals from being exposed to high concentrations of ozone during off periods and would greatly lengthen their life. Purging after generator shutdown would also prevent the harmful diffusion of ozone backwards into the inlet tubing, flowmeter, moisture and flow sensors, air filter and other air preparation equipment located in the gas supply line upstream of the generator. Although a check valve immediately before the generator could prevent this back-diffusion, the cracking pressure of the check valve would have to be high enough for it to be able to reseat itself since there is no backpressure to assist such seating. The cracking pressure required would lower the operating pressure inside the ozone generator, thereby decreasing its output of ozone.
In systems employing a venturi, the gas flow is motivated by a water pump, and the obvious way to purge an ozone generator operating with a venturi would be to include a switch which turns off the power to the generator some time interval before the water pump goes off. There are two disadvantages to this: 1) one cannot anticipate the shut-down of the water pump in all situations; and 2) in applications such as spas operating with ozone as the sole disinfectant, where the flow of water usually carries high concentrations of bacteria out of the filter (which bacteria are normally immediately killed by the addition of ozone and therefore never reach the spa), it is undesirable to run the circulation pump for even a few seconds without the ozone generator being on.
An additional problem in managing shutdown in prior art ozone generation systems which employ a venturi to inject ozone into water concerns the check valve needed in the ozone delivery line to prevent water from backing up towards the generator after shutdown (or, in situations where the point of ozone injection is above the water level of a vented vessel such as a tank, pool or spa, to prevent air from being sucked through the valve in the forward direction which would cause the plumbing to drain into the vessel and thereby lose its prime). In the majority of systems there is no backpressure after shutdown to firmly re-seat the check valve. Thus for reliable operation, the cracking pressure must be rather high, and this high cracking pressure greatly decreases the efficiency of the venturi which must draw ozone through the check valve.